In situ synthesis and electrochemical corrosion behavior of plasma electrolytic oxidation coating containing an osteoporosis drug on AZ31 magnesium alloy

In-situ synthesis of a pure, homogenous coating containing osteofos drug on an AZ31 Mg alloy substrate was reported via a one-step plasma electrolytic oxidation process in a phosphate-based electrolyte using a soft sparking waveform. FESEM/EDX, GIXRD/XRD, ATR/FTIR, XPS, AFM, ICP/OES, static contact angle measurement, OCP, and EIS characterization techniques were used. The coating demonstrated enhanced hydrophilicity and crystallinity, improved general corrosion resistance, reduced porosity, compact morphology, and decreased roughness. AFM technique revealed more peaks than valleys and a more symmetrical height distribution in the drug-loaded coating. XPS technique identified the N 1s (∼401 eV) peak consisting of M − NH₂ (∼397 eV), M − N (∼399 eV), and P–N (∼402 eV) bonds confirming the formation of a bisphosphonate/alendronate monolayer. Alendronate molecules were chemically bonded to the coating's outermost layer through –NH₂ amino and P–O–H polar heads. Due to the coatings' dual-layer structure, an equivalent electric circuit model with two parallel time-constants simulated the EIS profiles up to 72 h of immersion in simulated body fluid. An inductive loop in the low-frequency end of Nyquist plots indicated sustained drug release and progressive biodegradation within 48 h and 72 h of immersion. Kinetic control was the rate-controlling step in the electrochemical interface.